Frequency conversion, either up-converting from a lower frequency (longer wavelength) to a higher frequency (shorter wavelength), or the opposite down-conversion, is used to generate output radiation from input radiation of different frequencies to cover gaps in spectral regions of interest. For example, Terahertz (THz) radiation is of great interest for communication and imaging applications and shows promise for ultra-wideband wireless communications, homeland security, medical imaging, and defense imaging applications, among others. Due to their high sensitivity and selectivity, THz-based systems can be used to monitor public facilities, high-occupancy buildings, and even the open air for toxic industrial chemicals, chemical agents, biological agents, and trace explosives in a continuous and autonomous manner. In particular, because of its superior ability to penetrate through many materials, THz radiation is well-suited for the detection and imaging of chemical and biological weapons concealed under clothing. In addition, wavelengths (e.g., 10 microns-3 millimeters) in the THz range (e.g., 0.1 THz-30 THz) may resonate with many biological molecules, including strands of DNA, in a unique manner. As a result, THz sources may also be used as sensors for the early detection of bioaerosols such as spores, bacteria, viruses, and pathogens.
The ready availability of powerful visible and near-IR laser sources makes second order nonlinear optical processes an attractive mechanism for producing THz radiation. In particular, difference frequency generation (DFG), in which coherent mixing produces the THz radiation field, E(ω3)∝E(ω1)E*(ω2), from two input pumps, can be used to produce a spectrally pure, potentially tunable, room temperature THz light source. Unfortunately, the lack of sufficiently powerful compact THz sources and detectors, particularly in the 0.3 to 30 THz range, has drastically limited the development of THz sources for use in many applications.